Investigations into the unintended effects of vaccines tend to focus on potential adverse events, but some studies in low-income countries have shown that widespread immunization can cause unexpected positive consequences. A Danish team has now generated evidence that some of these unintended benefits are seen in wealthy countries, too.
The researchers set out to assess whether the live vaccine against measles, mumps, and rubella (MMR) is linked to lower rates of hospital admissions for infections. After scouring the vaccination and hospitalization records of almost 500,000 children, the researchers concluded that the MMR vaccine--which is sold by Sanofi ($SNY), Merck ($MRK) and GlaxoSmithKline ($GSK)--is associated with a lower rate of admission. The trend was particularly pronounced for lower respiratory tract infections and long hospital stays.
However, when MMR was given after the diphtheria, tetanus, pertussis, polio and Haemophilus influenzae type b (DtaP-IPV-Hib) vaccine, no correlation with hospitalizations was seen. The finding suggests that parents who create their own immunization schedules, for instance by delaying the three DtaP-IPV-Hib vaccines normally given in the first year of a baby's life, risk losing the unintended benefits of the MMR shot. In the Danish data set used in the study, about 50% of children received the MMR vaccine on time.
The researchers believe increasing this number could prevent respiratory-related hospitalizations, as well as cut cases of measles, mumps and rubella. "Physicians should encourage parents to have children vaccinated on time with MMR and avoid giving vaccinations out of sequence, because the present study suggests that timely MMR vaccination averted a considerable number of hospital admissions for any infection between ages 16 and 24 months," the authors wrote in The Journal of the American Medical Association (JAMA).
While having all parents follow the recommended vaccination schedule is likely to reduce cases of measles, mumps and rubella, the effect on hospitalizations is more debatable. As two British researchers note in an accompanying JAMA editorial, confounding factors or bias could account for the trend seen in the paper. Randomized controlled trials designed to test the hypothesis would be needed to confirm that the MMR vaccine is having an effect on hospitalizations.